This invention relates generally to rotation sensors and particularly to ring laser gyroscope rotation sensors. This invention relates particularly to dither drive apparatus and methods for preventing mode locking in ring laser gyroscopes. Still more particularly, this invention is directed to a dither drive technique that uses computer-generated signals to control a set of piezoelectric torquers that dither a ring laser gyroscope.
A ring laser gyroscope employs the Sagnac effect to detect rotation. Counter-propagating light beams in a closed loop will have transit times that differ in direct proportion to the rotation rate of the loop about an axis perpendicular to the plane of the loop. The ring laser gyroscope uses the resonant properties of a closed cavity to convert the Sagnac phase difference between the counterpropagating beams into a frequency difference. In an active ring laser gyroscope the cavity defined by the closed optical path becomes an oscillator, and output beams from the two directions interfere to give a beat frequency that is a measure of the rotation rate. The high optical frequencies of about 10.sup.15 Hz for light used in ring laser gyroscopes cause the minute phase changes to become beat frequencies that are readily measured.
When the rotation rate of a ring laser gyroscope is within a certain range, the frequency difference between the beams disappears. This phenomenon is called frequency lock-in, or mode locking, and is a major difficulty with the ring laser gyroscope because at low rotation rates it causes a false indication that the device is not rotating. The range of rotation rates over which lock-in occurs is the deadband of the ring laser gyroscope.
Lock-in arises from coupling of light between the beams. The coupling results primarily from backscatter off the mirrors that confine the beams to the closed path. Backscatter causes the beam in each direction to include a small component having the frequency of the beam propagating in the other direction. The lock-in effect in a ring laser gyroscope is similar to the coupling that has long been observed and understood in conventional electronic oscillators.
Any inability to accurately measure low rotation rates reduces the effectiveness of a ring laser gyroscope in a navigational system. There has been a substantial amount of research and development work to reduce or eliminate the effects of lock-in and to enhance the effective use of ring laser gyroscopes in such systems.
There are several known approaches to solving the problems of lock-in. Various biasing techniques have been employed to avoid the dead band so that lock-in would not be a problem in ring laser gyroscope operation. Biasing techniques can be divided into mechanical and optical techniques and into fixed and dithered bias techniques.
One approach involves mechanically oscillating the ring laser gyroscope about its sensor axis so that the device is constantly sweeping through the deadband. This mechanical oscillation of the ring laser gyroscope is usually called dithering. A typical ring laser gyroscope may be dithered at about 400 Hz with an angular displacement of a few arc minutes.
The amplitude of the dithering must be carefully controlled and monitored to minimize the effects of lock-in. Since the dither oscillation angular velocity and displacement relative to a support structure can be constantly monitored, they may be excluded from the output signal of the ring laser gyroscope. However, it has been found that a constant dithering amplitude is inadequate to eliminate all of the effects of lock-in.
Another approach to reducing lock-in error is to superimpose a random signal upon the amplitude of the dither driving amplifier. A random bias technique is described in U.S. Pat. No. 3,467,472. Several rather severe disadvantages to the random bias technique have been found, however. The phase error, even though randomized by the technique described in this patent, is not eliminated and still remains a relatively large source of error.
When the sign of the frequency difference reverses, the two beams tend to lock-in since at some point the frequency difference between the beams is zero. Since the output angle of the ring laser gyroscope is generally derived from the frequency difference, which locks in to indicate a zero rotation rate even if the actual rotation rate is non-zero, an error accumulates in the output angle. The periods of time when the two beams are locked in are usually very short so that the resulting output angle error is very small for any single sign change. Nevertheless, the error resulting from lock-in during sign reversal of the frequency difference is cumulative, and in time may become significant, particularly in precision navigational systems. This error is usually the major contributor to the random walk or random drift.
U.S. Pat. No. 4,529,311 to Morgan et al. is directed to a dithered ring laser gyro system in which the phase relationship between a pair of beams is accounted for. This phase may be used in a feedback loop for error control or it may be utilized to generate a set of error parameters for error correction. Morgan et al. regards the phase offset and the coupling efficiency of the two beams as being independent of time and temperature. However, the phase offset and the beam coupling efficiency are time and temperature dependent, which limits the accuracy of the error correction disclosed in Morgan et al.
U.S. Pat. No. 4,248,534 to Elbert is directed to the elimination of errors induced in dithered ring laser gyroscopes. Elbert discloses the use of a regression algorithm for minimizing lock-in. For a short time on both sides of zero velocity a trace of the rotation rate is stored in a computer memory. When there is no lock-in, this trace is a parabola. Deviations from the parabola are indicative of the lock-in rate.
U.S. Pat. No. 4,473,297 to Simpson et al. is directed to the use of phase differences between the alternating components in the counterpropagating beams to minimize lockin in a ring laser gyroscope. Signals indicative of the phase differences in the separate beams are input to a mirror driver circuit that drives two cavity length control mirrors to control the phase difference. Simpson et al discloses that the preferred phase difference between the beams for minimum lock-in is 180.degree..
In present dithered ring laser gyroscope systems, elaborate dither drive hardware is used to generate the proper frequency and amplitude for the signals applied to the piezoelectric transducers. A navigation system must include sensors for measuring rotations about three mutually orthogonal axes. Three ring laser gyroscopes may be mounted on an instrument block on three mutually orthogonal surfaces to measure rotations of the device in which the instrument block is mounted. Lines perpendicular to these surfaces define the sensing axes of the three gyros.
Unfortunately dithering one of the ring laser gyros can cause the sensing axes of the other two gyros to deviate from the desired lines. This deviation of the sensing axes is known as "coning" because the sensing axis of each gyro moves to define a cone centered on the line that defines the desired orientation of the sensing axis. U.S. patent application No. 740,371, filed June 3, 1985 discloses a dither drive system in which computer generated signals are used to reduce coning. That application discloses driving each gyro with a drive signal having a distinct frequency; generating a signal indicative of the coning motion of each gyro; demodulating the signals to determine the components of the motion that are not about the desired sensing axis; deriving virtual drive signal components corresponding to the the off-axis components; and combining the virtual drive signal components with the drive signals to reduce the off axis-components.